Attraction and repulsion have different definitions. You might have heard before that going to the beach is an attractive idea for people during the summer or how rotten food has a repulsive smell. However, in physics, we use these terms to describe the behaviour of non-contact forces. Did you know that water can be attracted to a plastic comb? Try this, grab a plastic comb and pass it through your hair several times. Now, let the faucet water run, forming a thin stream and try bringing the comb close to it.
A comb can attract water after using passing it through your hair. StudySmarter Originals
Were you surprised? This happens because two objects can interact without touching resulting in a tendency to move closer - attraction - or to go away from each other - repulsion. We see this behaviour in charged bodies and magnets. In this article, we will explore how this works in more detail to understand when electric and magnetic forces are attractive or repulsive and how we can calculate electric force.
Meaning of attraction and repulsion in physics
As we mentioned before in physics, there is a special type of force called non-contact forces.
Non-contact forces are interactions that can affect a body's motion without the need to touch it.
Examples of non-contact forces include gravity, electrostatic, and magnetic forces.
Non-contact forces can be attractive or repulsive.
A force is attractive if the objects interacting exert a pull on each other, creating a tendency to move closer. On the other hand, a force is repulsive if the interacting objects push each other away.
Gravity is always attractive. That is why no matter where we are we feel a pull 'downwards' towards the centre of the earth. However, electrostatic and magnetic forces can be both attractive or repulsive depending on the charge of the objects involved or the pole of the magnets.
Attraction and repulsion of charges
An electrostatic force is an interaction that occurs between charged particles.
There are two types of charges: positive and negative. If two objects have the same type of charge the electrostatic force between them will be repulsive. This means that if a positively charged object gets close to another, they both will push each other away. The same occurs for two negatively charged objects.
Objects with the same charges feel a repulsive force, StudySmarter Originals
On the other hand, opposite charges attract each other. So when one positively charged object is near a negatively charged object, they pull towards each other, tending to move closer.
Objects with opposite charges feel an attractive force, StudySmarter Originals
Law of attraction and repulsion of electrostatic forces
In 1785, Charles-Augustin de Coulomb, measured the magnitude of electrostatic forces between charged objects using a torsion balance that he invented. He used a needle with a brass disc on one end and a counterweight on the other. The needle was suspended by a fibre so it could turn. He then charged a metallic sphere and the brass disc with the same type of charge and measured how far apart they pushed each other away by using a scale attached to the outer part of the device.
After repeating this process for different amounts of charges, he was able to come up with a formula to quantify the strength of electrostatic forces. That formula is now known as Coulomb's law and it depends on the charge of the objects and the distance between them.
In this formula, the forceis measured in newtons,is the charge of the first object in coulombs, similarly,is the value of charge for second object, andis the distance between them, measured in metres. The constantis called coulomb's law constant and it depends on the medium that the charged objects are in. For example, if the objects are in a vacuum the value ofis approximately.
Attraction and repulsion between magnets
A magnet is any object or material that produces a magnetic field.
Regardless of its shape, every magnet has two poles called north (N) and south (S) poles. Similarly to the case of electric charges, unlike magnetic poles attract.
Opposite poles of two magnets feel an attractive force, StudySmarter Originals
Furthermore, same magnetic poles repel each other.
Same poles of two magnets feel a repulsive force, StudySmarter Originals
The earth is like a giant magnet itself, but its geographic north pole is really a magnetic south pole. Similarly, the geographic south pole is a magnetic north pole. Because of this, a free-to-move magnet - the needle of a compass, for instance - will always align its magnetic north pole to the earth's geographical north pole, and the magnet's south pole aligns with the earth's geographic south pole. This is why magnets' poles are named that way.
Magnets work due to magnetic forces that occur in atoms forming magnetic zones in the material that act like tiny magnets themselves. Usually, these zones are randomly aligned. However, there is a special type of material called ferromagnetic. They have the property that their magnetic zones align easily when exposed to an external magnetic field, and they maintain this alignment after the magnetic field is gone, becoming magnets themselves! An object that generates a magnetic field due to its internal structure is called a permanent magnet.
Ferromagnetic materials are strongly attracted to magnets. Some examples are iron, cobalt, and nickel.
Examples of attraction and repulsion
Now that we understand how electrostatic and magnetic forces work, we will explore some daily examples where we can observe attractive and repulsive forces in action.
Examples of attraction and repulsion: Electrostatic forces
A great and very common example involves static electricity. A balloon can be negatively charged by rubbing it against your hair. When rubbing, lots of electrons 'jump' from the hair to the balloon. This results in the hair being positively charged due to the lack of electrons, and since the balloon gained those electrons, it is now negatively charged.
A balloon can be negatively charged by rubbing it against your hair, StudySmarter Originals
Now, the balloon is attracted to the positively charged hair. The electric force can be seen at a distance when the balloon is moved closer to the hair - the hair is pulled towards the balloon's surface. Both the hair and the balloon are attracted to each other.
The balloon attracts the positively charged hair, StudySmarter Originals
Similarly, if two balloons are rubbed against some hair, both of them get negatively charged. If they are put near each other, they push each other away.
Two negatively charged balloons repelling each other, StudySmarter Originals
As you might have deduced, this is the same reason why the comb attracts the stream of water after you pass it through your hair, it obtains an electrostatic charge!
Examples of attraction and repulsion: Magnetics forces
As we explained before, you can see attraction and repulsion due to magnetic forces when playing with two magnets. However, if you just have one magnet, you can still see how this will attract some metals. In particular, iron, cobalt, and nickel are strongly attracted to magnets since they are ferromagnetic.
A magnetic field can be generated by a current flowing through a conductor. This will make the conductor behave like a magnet. By looping a wire over a metal core and connecting it to a battery we create an electromagnet. The end connected to the positive terminal of the battery will act like the south pole of the magnet, and the end connected to the negative terminal will behave like the north pole.
A current flowing through a wire can turn it into an electromagnet, StudySmarter Originals
Attraction and Repulsion - Key takeaways
- Attraction and repulsion are characteristics of non-contact forces such as electrostatic and magnetic forces.
- Non-contact forces are interactions that can affect a body's motion without the need to touch it. Examples of non-contact forces include electric and magnetic forces.
- Objects having opposite charges experience an attractive electrostatic force, pulling them towards each other.
- Objects having the same charge experience a repulsive electrostatic force, pushing them away from each other.
- Like poles of two magnets experience an attractive magnetic force, pulling them towards each other.
- Opposite poles of two magnets experience a repulsive magnetic force, pushing them away from each other.
- Coulomb's law states that the force exerted between two charged objects, whether a force of attraction or repulsion, can be determined by the charge of both objects, the distance between them, and the medium in which the charges are.
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